Industry needs to reach consensus soon on
whether immersion or EUV reigns at 32 nm
The field has been narrowed down to three choices as the industry grapples with its lithography options for how to shrink the k1 value enough to succeed with 32-nm half-pitch process technology by a 2009 target date. Two alternatives involve an extension of ArF immersion lithography (IML). One of those options is to continue using water and a reasonably high numerical aperture, and gain the necessary image enhancement through the use of double patterning. The second immersion-extension scenario involves the development and introduction of highly refractive fluids and higher-NA lenses, which together would push the numerical aperture to around 1.5. The third option involves the implementation of an entirely new lithographic projection platform and supporting infrastructure—extreme UV (EUV). Significant investment is ongoing for all three technologies at the stepper, track, resist, and other supply-chain companies as well as at the chipmakers and consortia.
Several industry events at the tail end of 2005 addressed the 32-nm and beyond lithographic conundrum. Participants weighed in at Cymer’s 13th annual Japan Lithography Symposium and JSR Micro’s Seminar Japan (both held in conjunction with Semicon Japan), as well as at the International Symposium on EUVL, which took place in San Diego in November. Several key supplier companies also offered insights into the state of developments during meetings on the Semicon Japan show floor.
Whatever choices are made for 32 nm, the era of production-worthy immersion lithography for 65 and 45 nm has arrived, boosting its chances as a candidate for even-smaller half-pitch production scenarios.
Peter Jenkins of ASML told MICRO at Semicon Japan that the company had shipped nine scanners, with the shipment of another one imminent and 10 more systems in backlog. He said the company plans to ship 20–25 IML tools in 2006. During MICRO’s early-December visit to the Nikon plant in Kumagaya, marketing staffer Andy Hazelton said that they’ve received “tens of orders” for the company’s NSR-S609B ArF immersion tools, adding that the production floor in the factory was very busy getting the systems ready to ship.
In his talk on the current status and future outlook of 193-nm IML at the JSR Micro event, IMEC’s Kurt Ronse noted how great strides had been made in understanding and reducing immersion-related defects and resist leaching—although a good deal of work remains to be done.
A series of charts detailing research results at IMEC revealed progress in the areas of air bubble–induced defects, offered examples of how contact angles correlate with defectivity, and presented champion data offering proof of concept of low-defectivity IML. Ronse also showed examples of how immersion resist leaching has been improved but added that topcoats still must be employed, since they reduce leaching by at least an order of magnitude. Overall, immersion-specific resists have started “to show similar lithographic performance as dry ArF resists.”
In conclusion, Ronse said that “ArF immersion is still on track” for high-volume manufacturing at the 65-nm half-pitch insertion point and that “confidence exists that ArF immersion based on water will also serve the 45-nm half-pitch node.”
On the EUVL side, new rankings compiled at the San Diego symposium reveal that the development of effective photoresists now tops the list of critical issues for enabling EUV-based manufacturing in 2009. In the new poll, chipmakers and suppliers factored in the difficulties of simultaneously achieving resist resolution, sensitivity, and minimal line-edge roughness. Resists top both defect-free photomasks (which dropped from first to third on the list) and collector optics, which held on to the number two slot. Other top issues include source power; reticle protection during storage, handling, and use; and quality and lifetime of projection and illuminator optics.
The Cymer conference included several presentations as well as an industry panel made up of representatives from several chipmakers, the three stepper manufacturers, the dominant track tool company, a top maskmaker, and a leading photoresist supplier. While they were not necessarily in agreement about which of the three technologies vying for the 32-nm half-pitch node was optimal—an informal poll at the end of the panel rated IML double patterning and EUVL as the front-runners—the various participants agreed that the industry must soon come to some sort of consensus about which approach to take.
Sony’s Hiroichi Kawahira listed four critical areas that must be addressed at the 32-nm half-pitch node during his invited talk as well as in his comments as a panelist. In the optics area, he cited the stability of new catadioptric optics, the control of polarization, the development of a high-index liquid, and bi-refringence as key challenges. In terms of process integration, he wondered about how far single exposure or patterning can be extended, the development of new bottom antireflective coating designs and their integration, defectivity, and linewidth roughness. Birefringence and mask structure topped his photomask concerns, while the development of more-precise simulation and modeling as well as better coupling with design for manufacturing made his layout-optimization subcategory list.
The subject of high-index fluids came up during a panel exchange elicited by moderator Nigel Farrar of Cymer, who observed: “We’ve heard a lot about high-index fluids being needed for possible extensions of immersion. It seems to me that the immersion technology has come along so fast because the fluid that’s being used is water, and we have a lot of experience in handling water and purifying it and controlling it…so how difficult is it really to introduce a totally new fluid and have to learn how to deal with purity and manufacturing and so forth?”
Nikon’s Masahiro Nei replied through a translator that “from the tool manufacturer, if that fluid is provided or supplied between the lens, then we have to have development of the nozzle, which will continuously support and hold that liquid in between the wafer and the lens. I think we are far from this stage of something that is readily usable.”
“With high-index liquid, the most important parameter is the cost,” answered Mitsuru Sato of Tokyo Ohka Kogyo (TOK) in translation. “The immersion water and also high-index liquid water is less costly, so to speak, since you can recycle it and just dump it. However, if you go for high-index liquid, let’s assume some kind of substance is contaminating and dissolved into that liquid; you have to remove it in order to dump the liquid, like recycling. So can you just throw it away without incurring any cost?”
Farrar also commented that “developing new materials for lithography
applications has already been with the industry, and we have already seen how challenging that was with 157 nm. I think it’s a very significant barrier we face, with the additional challenge from an infrastructure standpoint of trying to realize these materials with a relatively short development phase for potentially only one node of application.”
Sony’s Kawahira underscored those remarks, pointing out some of the inherent difficulties in developing high-index refractive fluids. “If the index rises, then the absorption coefficient of the heat will change. So depending on that, to what extent will thermal loss be generated? Focus position may change, because if the material changes, then depending on the absorption during exposure, even with simulation, the focus may change by about 10 nm or so. So this type of analysis will have to be conducted, and we would need to get the results as quickly as possible from the manufacturers.”
Although defect-free masks slid a few slots down in the recent EUVL challenge rankings cited above, they remain one of the key obstacles in the development path of the new patterning technology. Cymer symposium panelist Naoya Hayashi of Dai Nippon Printing shared some recent work done by his company in conjunction with Japan’s Association of Super-Advanced Electronics Technologies (ASET).
“For EUV-specific items in the mask process development, we have evaluated reflectivity with the cooperation of ASET,” he said through a translator. “Multilayer reflectivity was measured, and buffer and absorber was included, and once we remeasured this, we found that the reflectivity fell by about 0.9%, but there is no shift of the central wavelength. Therefore, if the initial evaluation is such, this should be acceptable…. In order to repair the defect, [we used] AFM nanomachining repair technology… and this was printed by ASET.
We found that the defect size is still large; however, with the present repair technology, this [approach] is still viable.”
The advanced lithography debate will continue in the coming months at the annual SPIE International Symposium on Microlithography the week of February 19 in San Jose and the Sematech-sponsored Lithography Forum, Resist Summit, and related workshops taking place April 24–27 in Vancouver, BC.—TC
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© 2007 Tom Cheyney
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